In microelectronics, chemically amplified resists are used extensively for optical lithography and electron-beam writing (see "Solid State Technology," Vol. 34 (1991), no. 8, pages 53 to 60). Chemical amplification is utilized both for wet-developable single-layer resists and for entirely or partly dry-developable resists. In this context, the resists can operate on the principle of acid-catalyzed cleavage, wherein polar but blocked chemical groups, for example carboxyl groups or phenolic hydroxyl groups, are unblocked by means of a photolytically generated acid, and the resist changes polarity in the exposed regions. This change in polarity can be used, for example, to develop the resist in an alkaline developer or--in the case of dry-developable resists--for selective silylation. Examples of blocking groups are tert-butylester and tert-butoxycarbonyloxy groups.
U.S. Pat. No. 5,384,220 discloses a photolithographic pattern generation system in which a dry-developable resist is subjected, after exposure, to a thermal treatment (PEB=post exposure bake), then silylated from the liquid phase and then anisotropically etched or dry-developed in oxygen plasma. Depending on the type of silylation solution, positive or negative patterns (images) are produced. The resist generally consists of at least two solid constituents, i.e. a base polymer and a photoactive acid former. The base polymer contains carboxylic acid anhydride and tert-butylester partial structures, and the acid former is preferably an onium compound, such as diphenyliodonium and triphenylsulfonium trifluoromethanesulfonate. A resist of this kind is suitable in particular for photopatterning in the submicron and subsemimicron range with very steep slopes.
The "delay time effect" can be observed in a pattern generation procedure as outlined above--as with other known resist systems which operate on the principle of acid-catalyzed cleavage. Specifically, when the time span (delay time) between exposure and thermal treatment (PEB) exceeds a certain value, definite discrepancies then occur between the nominal pattern dimension (pattern size on the mask) and the pattern that is imaged (pattern size in the resist after development). The longer this delay time, the greater the discrepancies. Above a certain value for the delay time, for example about 30 minutes for anhydride-group-containing resists of the kind mentioned above, almost no patterns are recognizable after development. The tolerable delay time for these resists is approximately 5 to 10 minutes. For production-engineering reasons, however, such a short delay time cannot be accepted.
The aforesaid problem is commonly known, and is attributed to alkaline contamination in the air which deactivates the photochemically generated strong acid during the delay time. It is therefore proposed to solve this problem by filtering the air with active carbon (see "Proceedings, SPIE" Vol. 1466 (1991), pages 2 to 12). This requires large investments, however.
Even with other actions, for example by adding additives, it has not been possible to decisively attenuate the delay time effect (see "Proceedings, SPIE" Vol. 1466 (1991), pages 13 to 25). It is possible to extend the delay time by applying an additional layer, but only slightly. This action moreover constitutes an additional process step, which is, however, undesirable in production because it leads to yield losses.
For pattern dimensions&lt;0.25 .mu.m, which are becoming increasingly significant, diffusion of the photoacid generated by exposure, during the time period between exposure and PEB, is disruptive. The reason is that the acid diffuses from exposed to unexposed areas, so that those areas also become soluble; this, however, is undesirable. In addition, alkaline contaminants can diffuse into exposed areas and deactivate the photoacid there. The result of both effects, however, is that pattern dimensions which are on the order of the diffusion length can no longer be generated reliably.
The lateral diffusion effects cannot be reduced by any of the actions mentioned above. A reduction in diffusion can be accomplished by generating a denser polymer structure (as a result of an elevated temperature when evaporating the solvent from the resist solution) (see "Journal of Photopolymer Science and Technology" Vol. 8 (1995) No. 4, pages 643 to 652), but this action requires special polymers with a low glass transition temperature, and is therefore not generally applicable.